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July 2017 night sky guide

July 3, 2017

By Andrew Jacob

To help you learn about the southern night sky, Sydney Observatory provides an audio guide/podcast, transcript of that audio, and a sky map or chart each month. This month’s guide is presented by Dr Andrew Jacob, Sydney Observatory’s Curator of Astronomy.

This month, learn how to find the winter constellations. Take the sting out of cold winter nights by identifying the Scorpion in the sky and searching for star clusters in the Milky Way. Watch the dance of Jupiter’s moons and see Venus replace the eye of the Bull, Taurus.

Hear the Audio
You can subscribe to the podcast on iTunes or directly download this months guide to your favourite audio listening device.

See the Sky Chart
We provide a July 2017 night sky map (PDF) which shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.

Read the Guide

Hello, and welcome to the Night Sky Guide for July 2017.

This is Andrew Jacob, and I’m the Curator at Sydney Observatory, part of Sydney’s Museum of Applied Arts & Sciences. Come on a tour of the night sky with me. Learn what stars and constellations are visible, where to find the planets and what special events are happening overhead this month.

To make the most of this guide, you should begin by gathering a few items together. Firstly, you will need a star map. You can download a free one from Sydney Observatory’s website where you’ll find it in the Astronomy Resources section under Monthly Sky Guides. The star map will show you what stars and constellations are visible in the night sky this month and I’ll be referring to that star map in this guide.

As well as the star map, a torch with a red LED, or one covered with a few layers of red cellophane, will be very useful. The red light will allow your eyes to remain dark adapted during the evening, yet still allow you to read your star map.

Finally, a pair of binoculars or a telescope can be very handy. They’re not essential for following this guide but if you do have them they will help you see a few of the fainter objects more easily and in more detail.

Now that we have our equipment together we need to know a few directions and also how to measure angles across the sky.

You can find the cardinal directions – North, South, East and West – from a compass app on your mobile device, or just remember, of course, that the Sun rises in the east and sets in the west. And if the Sun is setting at your right shoulder, then you must be facing south. Another useful direction to remember is the zenith. This is the point directly overhead.

To find your way around the night sky, it helps to know how to measure angles across it. It makes no sense to say, for instance, that one star is “2 centimetres” to the left of another or that a shooting star left a trail “half a meter” long! Instead we should use angular measurements. The distance around the horizon, from North, through East, South, West and back to North is 360 degrees. And from the horizon vertically upwards to the zenith overhead is 90 degrees.

But how do we measure smaller angles? Well, despite the great variety of human form our fingers, hands and arms are all pretty much in the same proportions. If you hold your hand out at arm’s length and stretch out your little finger and thumb to make a hand span, they span an angle across the sky of about 20 degrees. A fist held out at arm’s length makes an angle across the sky of about 10 degrees. And a finger held up at arm’s length is about 1 degree, or twice the width of the Moon or the Sun. I’ll be using these measurements during this guide. So, a hand span at arm’s length is 20 degrees, a fist is about 10 degrees, and a single finger is about one degree across the sky.

One final point to note before we get started. The Earth rotates and so the sky changes hour by hour. My descriptions of the constellations and stars in this guide fit the time of one to two hours after sunset.

And in July the nights are long and cold so please dress for the occasion!

Let us begin our tour of the night sky by looking towards the south. If you’re facing south in the early evening and you look high in the sky, 60 degrees or 3 hand-spans up from the horizon, you should be able to see two bright stars, side by side. If you’re in a bright, light‑polluted location, such as central Sydney, they may be the only two stars you can see in that direction. Away from the city and other bright lights, they will be the brightest pair of stars you see towards the south.

These two bright stars, side by side, are known as “The Pointers,” so-named because they help us find the Southern Cross by pointing towards it.

On your star map, the Pointers are clearly labeled. The Southern Cross is officially known as Crux, which is Latin for cross. The brighter of the two pointers is called Alpha Centauri. The other one is called Beta Centauri. Alpha Centauri is the third brightest star in the night sky, while Beta Centauri is the 10th brightest star.

Alpha Centauri is a very interesting star. Although it looks like a single star to your eye, it is, in fact, a group of three, two stars are so close your eye cannot separate them and the third is too faint to see. Through telescopes two of the stars are sometimes visible. Both are very similar in size and colour to our Sun. They’re orbiting about each other with each orbit taking about 80 years. The third star in the system is called Proxima Centauri and it is the closest star to the Earth, after our Sun of course, but it is too faint to see by eye.

Proxima Centauri is a red dwarf star and is only visible through large telescopes. It is believed to be orbiting the first two stars. Proxima Centauri is about 4.2 light‑years away, or approximately 42 million million kilometers.

Recently a planet was detected in orbit around Proxima Centauri. This makes it the closest planet beyond the eight in our own solar system – a very tempting target to send the first interstellar spacecraft to! But that is for the future.

A moment ago I mentioned “light‑years”. What is a light year? It’s a measure of distance, even if it sounds like a time. If you have a torch shine its light into the sky for a moment. The light from your torch travels incredibly fast. In just one second it goes almost 300,000 kilometers. That’s seven and a half times around the Earth or almost the distance to the Moon. In fact in just one and a quarter seconds your torch light would reach the Moon, 380,000 kilometres away. In eight and a half minutes the light would reach the Sun, 150 million kilometres from Earth. After five hours light from your torch would pass Pluto. Finally, after one year of time your torch light will have travelled one light-year of distance. Yet we still haven’t reached the next nearest star!

Proxima Centauri, our closest star after the Sun, is about 42 million million kilometres away from us. Your torch light would take about 4.2 years to reach it. So we can say that Proxima Centauri is about 4.2 light years away from Earth. This also means that we see Proxima Centauri, the star, as it was about 4.2 years ago. We are looking into the past to see everything in the universe.

So a light year is simply a distance, and one lightyear is about 10 million million kilometers long.

Now, let’s get back to the night sky. We’ll return to Alpha Centauri. If you draw an imaginary line from Alpha Centauri to the right, through Beta Centauri (the other Pointer star), and onward, you will reach the Southern Cross. In July it’s at its highest point in the sky, standing upright and looking just as it does on the Australian flag.

The Southern Cross is very useful, as it can help us find the direction of true south. Hold your hand out at arm’s length, and measure the length of the long arm of the Cross using two fingers, from the top star of the Cross to the bottom star. Now, measure this distance downwards four times in the direction the long arm of the Cross is pointing.

You should find yourself pointing at the sky about half way between the Cross and the horizon. This point is called the South Celestial Pole, the south pole of the sky. It’s the point in the sky about which all the stars appear to rotate, but there are no bright stars here. It’s just an imaginary point in the sky.

Now, if you drop a vertical line from the South Celestial Pole straight down to the horizon, you’ll find the direction of true south on the ground.

Here is something interesting you can do if you have a camera that allows you to leave the shutter open (or an app that allows you to make ‘star-trail’ photos). If you take a photograph of the sky to the south, including the South Celestial Pole, and leave your camera shutter open for 10 minutes or more, you’ll find beautiful, circular star trails in your photograph.

Look back at the Southern Cross. The brightest star of the cross, the one at its foot, is called Acrux. It is labelled with the Greek symbol alpha (α) on your star map. Moving clockwise around the Cross we come to Mimosa, then at the top of the Cross Gacrux and finally on the right is delta Crucis.

Acrux, at the foot of the Cross, is another multiple star system like Alpha Centauri but it is about 320 light years away from us.

Mimosa, on the left, is about 108 light years away. If you have binoculars use them to view Mimosa. Now, the star itself is not too interesting but scan just to its left and you will see a small sparkling group, or cluster, of faint stars. Astronomers call this type of object an “open cluster” and this one is named the Jewel Box. It is a group of young stars, not more than 15 million years old about 5000 light years away. This cluster looks much better through any telescope, when stars of different colours – red, yellow or just plain white – are seen.

If you have an eye for colour you may notice that Gacrux, at the top of the Cross, is orange – it is a cool red-giant star much larger than our Sun. It is only 88 light years away so is the closest of the four main stars of the Southern Cross.

The Southern Cross appears on the Australian flag, of course. The flag designers did a good job of representing the stellar Cross – the two axes are slightly skew and there is a fifth star, which appears smaller on the flag and is fainter in the sky than the four stars I have just named. This fifth star is called epsilon Crucis. It appears on your star map between bright Acrux and delta Crucis on the right.

Surrounding the Southern Cross is the constellation of Centaurus the Centaur. His front leg is The Pointer stars, his back arches over the Cross and his back leg hooks down to the right of the Cross. A Centaur is a half-man, half-horse creature holding a bow loaded with an arrow. His head & upper body lie above the Pointers, but if you can make out the features of a man’s head and torso here then I admire your imagination!

Just above the Centaur’s back is a wonderful object called Omega Centauri. It is labelled on your star map, but you will only see it by eye if you are in a completely dark site with no Moon up and far from any lights. From a bright, light polluted, area you will need binoculars to spot it. Omega Centauri is a “globular cluster” of stars, a ball-shaped group of several million stars, tightly packed and about 16,000 light years away. There are over a hundred of these globular clusters scattered around our Milky Way galaxy, but this one is the largest and brightest.

Well, that covers the southern sky. Let’s now turn to the west.

To orientate your star map to the western sky, rotate it so that the horizon labeled “West” is at the bottom.

The constellation of Leo the Lion, one of the constellations of the zodiac, lies very close to the western horizon. Its brightest star Regulus, meaning “little King”, is about a hand-span above the horizon at around 6:30pm, however it sets around 7:30pm this month. Regulus is a hot star a hundred times brighter than our own Sun and about 77 light years away.

Above Leo is another zodiac constellation, Virgo. This figure is associated with the virgin goddess Astraea of the Greeks and also with the Greek and Roman goddess of wheat and agriculture. Her human form is hard to make out in the sky but the constellation’s brightest star is Spica, at about 60 degrees, or three hand spans, above the western horizon. Spica is Latin for “ear of wheat” and Virgo holds this wheat, perhaps fresh from the harvest, reflecting the theme of agriculture but also of fertility.

The two stars, Regulus and Spica, lie on a line that crosses the sky called the ecliptic – you can see it drawn on your star map as a dashed line. This line marks the apparent path of the Sun through the sky and along this line you will also find the planets and our Moon – but more about them later.

Let us now turn to the north.

You will notice an orange-red star due north, about 37 degrees or just over one and a half hand spans above the northern horizon. This is the star Arcturus in the constellation Bootes the Herdsman. But once again his human form is difficult to make out!

Finally we turn to the east.

Face due east, hold out your arm and measure three hand spans plus a fist upwards, to make 70 degrees, above the eastern horizon or almost overhead. You should see a bright, orange‑coloured star. Again, to orientate your star map, rotate it so that the horizon labeled “East” is at the bottom. The map should now match the eastern sky in front of you.

I’ve noted the colours of several stars so far. But I remember having great trouble seeing colour in stars when I first began looking at the sky. So if you don’t see the colours I describe tonight, don’t worry.

This star is called Antares which means the ‘rival of Mars’ because of its reddish colour. It’s an enormous, red supergiant star, around 400 times the diameter of our sun. If you placed it where our Sun is, it would reach out through the solar system and engulf the Earth.

Antares is about 604 light years away from the Earth. It’s a star coming to the end of its life. It will eventually die by exploding in a cataclysmic supernova, destroying itself in the process. However, it’s so far away that this will have no effect on the Earth, although it will be a spectacularly bright sight. Unfortunately, it will also be a few hundred thousand years before this happens.

Antares is the heart of Scorpius, the Scorpion, one of the few constellations that really looks like its name. If you have your star map with you, hold it up towards the east with the east horizon at the bottom, as I described earlier, and locate the star Antares. Just above Antares, you’ll see a short arc of stars which represent the head and shoulders of the scorpion.

Look back through Antares and out to the right. You will see a curving arc of stars reaching across to the right, the scorpion’s body, then an arc of stars hooking down and back to the left, his tail. At the very end of his tail, you can see his sting quite clearly. Finally, here’s a constellation that really does resemble its name!

Let’s move on. Below the sting of Scorpius is the constellation Sagittarius, which is supposed to represent an archer in the form of a centaur, but I’ve never been able to see an archer, let alone a centaur, when I look at this set of stars. All I can see is a rather triangular teapot. On your star map, Sagittarius is highlighted as the Teapot. Can you see the Teapot pouring tea all over the tail of Scorpius?

Just off the tip of the Teapot is an interesting point in the sky. If you’re away from bright city lights and you’ve been outdoors for more than 15 minutes or so, and your eyes have adjusted to the darkness, you might have noticed the Milky Way stretching overhead. It’s a band of faint, milky light stretching from the Southern Cross past the Two Pointer stars and continuing overhead through the tail of Scorpius, through Sagittarius and finally down to the eastern horizon.

The center of our Milky Way galaxy lies just above the tip of the Teapot not far from the sting of Scorpius. It’s a good thing we’re out here near the edge of the Milky Way galaxy and a long way from its center. Because at the center of our galaxy, 27,000 light years away, lies a very large black hole over four million times the mass of our Sun. We’re quite safe from it here, out towards the edge of the galaxy.

If you have binoculars handy here is your observing challenge for July. Slowly scan the region of Sagittarius, Scorpius and over to the Southern Cross and beyond. In this region there are many open clusters (like the Jewel Box), globular clusters (like Omega Centauri), multiple stars (like Acrux and Alpha Centauri) and nebulae (like the Orion nebula we met in January’s Guide). It shouldn’t take long before you discover at least one of each for yourself!

Now we’ve completed our tour of the sky for July.

What are the special events and highlights in July 2017?

Let me note that all the times I mention are in ‘clock’ time, the time a clock would show or Eastern Australian Standard Time. Please make the appropriate adjustments for your time zone if necessary.

Let’s start with the Moon phases. The month begins with a First Quarter Moon on Saturday 1st at 10:51am. Full Moon falls on Sunday the 9th at 2:07pm. Last Quarter is on Monday 17th at 5:26am. New Moon occurs on Sunday, 23rd July at 7:46pm. And finally there is another First Quarter Moon on Monday 31st at 1:23am.

What planets are visible in July 2017?

This month’s showstoppers are the gas giant planets – Jupiter and Saturn. They’re easily seen by eye. And the best time to view them with a telescope is when they are high overhead in the evenings.

Jupiter lies in Virgo, not far from the star Spica, but it’s much brighter than Spica. Watch it through binoculars (or a telescope) and you will see its four largest moons – Io, Europa, Ganymede and Callisto – moving back and forth past Jupiter as we view their orbits from side-on. Io, the innermost of the four, takes just 42 hours to orbit Jupiter so you will see its position change in just a few hours during one evening. The other moons take from a few days to over two weeks to orbit Jupiter so you can watch them change position throughout the month.

The Juno spacecraft is also presently orbiting Jupiter. Designed to investigate the planet’s structure & composition it’s already turned our knowledge of Jupiter’s polar regions on its head. And during July it flies low over the Great Red Spot. Look out for some great close-up images.

Saturn lies in Ophiuchus this month and is well placed for observing. Also its rings appear at their maximum tilt towards us giving very nice views through telescopes. The Cassini spacecraft is presently in its Grande Finale orbits about Saturn, passing between the rings and the planet before ending its mission by plunging into Saturn’s clouds in September.

Mercury is visible shortly after sunset low in the north-west, initially in Gemini but racing through Cancer and into Leo as the month goes on. On the 25th, just after sunset, look for Mercury above a thin crescent Moon and beside the star Regulus making for a slightly twisted smiley in the western sky.

Venus still dominates the morning sky, rising from around 4am. It begins July in Taurus and moves into Orion. On July 13 Venus appears to take the place of one of the eyes of Taurus, the Bull. And on the 21st a thin crescent Moon lies below and right of Venus making for a striking sight.

Finally, this month Earth reaches its furthest point from the Sun, or aphelion, on July 4 at 6:11am. At that point we will be just over 152 million kilometres from the Sun.

If you’ve enjoyed this Night Sky Guide and want to regularly check out what’s in the night sky, why not purchase a copy of Sydney Observatory’s annual guide, “The Australasian Sky Guide.” It’s jam packed with monthly astronomical information, including rise and set times for the Sun, Moon, and planets, tide times and a detailed look at our solar system and upcoming astronomical events. It’s only $16.95 from Sydney Observatory or the MAAS store, or you can purchase it online, for which additional costs apply.

For more astronomical information, check our website and blogs at www.sydneyobservatory.com.au. Keep in touch via our Facebook and Twitter accounts.

And if you’re in Sydney visit the Observatory in the Rocks area. Join a tour to view the skies through our telescopes – day or night. Visit our Space Theatre or the Sydney Planetarium. Tour our exhibition and discover the history of Australian astronomy. But please check our website as not all the activities are free and some require bookings.

And that brings to an end this Night Sky Guide from Sydney Observatory and from me, Andrew Jacob. Thank you for listening and I wish you clear skies until next time.